This invention relates to a curable composition and especially to a curable composition that can have controlled cure through the use of appropriate curing agents.
It is known that printing plates can be manufactured by hardening ethylenically unsaturated compounds which become crosslinked and insolubilized under action of actinic rays in the presence of photosensitizers. In hardening such compositions to form printing plates at room temperature under action of actinic rays, addition polymerization of polymerizable double bonds in the compounds is inhibited by air. This inhibition causes a very fine liquid film to reside on the surface causing various degrees of tack. Consequently, it is presently generally considered desirable to replace the oxygen at the photosensitive surface with an inert gas, treat the surface with chemical additives that absorb or render the oxygen inactive, or use very intense light sources that literally burn away the oxygen.
It is a primary object of the present invention to provide a liquid or solid photo-crosslinkable composition for the manufacture of relief printing plates which substantially meet the following:
1. Even in the presence of air, image-wise exposure of the composition gives a completely tack-free surface, eliminating the need for subsequent post exposure treatment of the crosslinked areas forming the relief image or for expensive equipment for carrying out the exposure in the absence of oxygen.
2. The requisite exposure time for the composition should be as brief as possible in order to achieve a short cycle time for the manufacture of a finished printing plate as might be demanded by the newspaper industry.
There are a number of U.S. and foreign patents that show photocurable compositions that have as major chemically functioning constituent a polythiol. This is unlike the present invention wherein the thiol constituent seemingly acts in some way to regulate and make more effective the polymerization of the other constituent components of the composition. In one species of the generic invention the resin is C--C unsaturated and crosslinks with monomer as a major characterizing reaction with the thiol being in an amount to primarily act, in substance, almost like a scavenger to close off oxygen interference without changing the other basic properties of the system and the end product from what they would be without the thiol. The other species which has the binder that is C--C saturated has the thiol functioning in the same manner but it is the monomer that crosslinks with itself forming the latice with the binder filling the space between the crosslinked chains and being bound thereby. The binder earlier acted as a matrix to hold the crosslinking monomer in position to form a proper product. The curable compositions of the present invention as a general rule cure within minutes without the presence of the thiol although with the presence of the preferred polythiol the cure is usually even faster. Examples of patents that show photocurable compositions that have a polythiol as a major chemically functioning constituent are U.S. Pat. Nos. 4,008,341 3,843,572; 3,832,421; 3,783,152; 3,666,461; 3,661,744; 3,627,529; and 3,615,450 and Canadian Pat. Nos. 930,094; 926,183; 924,047 and 885,388. Also the general theory as represented at pages 22-24 has been in general known to scientists.
Many other patents independently reveal various features of the inventive composition. They however lack the overall advantage presented by the synergistic activity of the present new composition. For example U.S. Pat. No. 3,674,486 and British Pat. Nos. 1,366,769 and 1,395,822 discloses compositions having an A-B-A type block copolymer which is a constituent of some species of the present invention. Similarly A-B block copolymer compositions are disclosed in German Patent Publications DT OS 24 56 439 and DT OS 26 10 206. A liquid ethylenically unsaturated polyetherurethane composition capable of being polymerized by actinic light to yield a solid is shown in U.S. Pat. No. 4,057,431. French patent publication No. 2,103,825 reveals acrylonitrile-butadiene copolymer compositions also containing an acrylate monomer.